The present invention relates to semiconductor memories and, more particularly, to a dynamic random access memory wherein each memory cell is constituted by a transistor and a capacitor.
As is well known, a dynamic random access memory (to be referred to as a "dRAM" hereinafter) has a plurality of memory cells formed in a matrix manner on a substrate. Address and data lines extend along column and row directions, respectively, to designate read/write of data from/into a desired cell. Each cell typically consists of a MOS transistor which is switched on/off in response to an address designation and a MOS capacitor for storing data carriers supplied through the transistor.
In a dRAM having the above-mentioned structure, the level of a data signal read out from a cell directly depends on the maximum charge which can be stored in each memory cell of the dRAM. Therefore, the capacitance of the cell capacitor must be increased in order to increase the level of the readout data signal. The cell capacitance is determined in accordance with the thickness and area of a gate insulation film of the MOS capacitor in the cell. Therefore, in order to increase the capacitance of the capacitor, the gate insulation film must be made as thin as possible, and/or the area of the capacitor region must be increased as much as possible. However, there are limitations in manufacturing a thin gate insulation film, and an increase in the capacitor area results in degradation in cell packing density of the dRAM, thereby preventing manufacture of a dRAM having a high packing density.
Occurrence of "soft-error" is also a big problem in a dRAM having the above-mentioned structure. A "soft-error" problem means the phenomenon wherein radiation, such as u rays generated from a radioactive material (contained in a very small amount in a package), becomes incident on a silicon substrate to generate a large number of electron-hole pairs therein, and the electron-hole pairs cause noise, thereby causing malfunction of the dRAM. As a result, it has been very difficult to improve the maximum charge storage amount without degrading memory density.